1. Field of the Invention
This invention relates to a distance measurement apparatus for transmitting an electromagnetic wave beam such as a laser beam, receiving an echo, calculating the time interval between the moment of the transmission of the beam and the moment of the reception of the echo, and measuring, from the calculated time interval, the distance to an object reflecting the beam and causing the echo.
2. Description of the Related Art
There is a conventional distance measurement apparatus mounted on a vehicle. The conventional apparatus intermittently emits a laser beam into a predetermined angular range outside the body of the vehicle. The predetermined angular range is scanned by the laser beam. Thus, the predetermined angular range is also referred to as the scanned range. When an object in the scanned range reflects the laser beam, a portion of the laser beam returns to the apparatus as an echo. The apparatus measures the time difference between the timing of the emission of the laser beam and the timing of the reception of the echo. The apparatus calculates the distance to the object on the basis of the measured time difference. The apparatus recognizes the direction of the emission of the laser beam which is returned as the echo. The apparatus detects the direction of the object relative to the vehicle in accordance with the recognized direction of the emission of the laser beam.
As the power of the emitted laser beam rises, the measurable distance to an object increases. A laser beam having an excessively high power adversely affects human eyes when being incident thereto. As a laser diode is driven at a higher power to generate a stronger laser beam, the life of the laser diode shortens.
Japanese patent application publication number 7-134178 discloses an on-vehicle distance measuring device using a laser beam which is designed to control the power of the laser beam to perform a proper measurement and avoid a bad effect on human bodies. In the distance measuring device of Japanese application 7-134178, the power of the laser beam is reduced as the distance to a target to be measured decreases. Also, the power of the laser beam is reduced as the relative speed between the target and the vehicle or the speed of the vehicle decreases. Therefore, the measurable distance to a target is short when the relative speed between the target and the vehicle or the speed of the vehicle is low.
Japanese patent application publication number 9-197045 discloses an on-vehicle radar device using a laser beam which is designed to control the power of the laser beam to avoid a bad effect on human bodies. Operation of the radar device in Japanese application 9-197045 is alternately changed between a preliminary scanning mode and a main scanning mode. During the preliminary scanning mode of operation, a predetermined angular range outside the body of the vehicle is scanned by the laser beam having a low power. A memory is loaded with preliminary data representative of laser beam transmission directions and distances to detected targets which are available in the preliminary scanning mode of operation. During the main scanning mode of operation which follows the preliminary scanning mode of operation, first portions of the predetermined angular range are scanned by the laser beam having a high power while second portions thereof are inhibited from being scanned. Specifically, the preliminary data in the memory are referred to, and suspension is given of the transmission of the laser beam in the directions in which targets in short distances have been detected during the preliminary scanning mode of operation. On the other hand, the laser beam having a high power is transmitted in the directions from which detected targets have been absent during the preliminary mode of operation.
In the radar device in Japanese application 9-197045, there is a timing difference between the preliminary scanning mode of operation and the main scanning mode of operation. During the preliminary scanning mode of operation, the predetermined angular range is fully scanned, and the preliminary data are stored into the memory. The timing difference, the fully scanning, and the storing of the preliminary data cause a slow response characteristic of the radar device.
Japanese patent application publication number 11-94945 discloses an on-vehicle laser radar device which scans a predetermined angular range outside the body of the vehicle by a train of pulses of a laser beam. In the radar device of Japanese application 11-94945, the number of pulses of the laser beam per unit time is changed depending on the speed of the vehicle or the distance to a measured object. Specifically, the number of pulses of the laser beam per unit time is set to a reduced value when the speed of the vehicle is equal to or lower than a prescribed value. The number of pulses of the laser beam per unit time is set to a reduced value when the distance to a measured object is equal to or shorter than a prescribed value.
Japanese patent application publication number 11-148974 discloses a distance measurement apparatus which transmits a train of pulses of a laser beam as forward pulses. When the forward pulses encounter an object and are reflected thereby, echo pulses occur correspondingly. The distance measurement apparatus of Japanese application 11-148974 includes a processor for detecting whether or not the amplitude of every received echo pulse is greater than a prescribed threshold value. The forward-pulse amplitude is increased as time goes by. During the increase in the forward-pulse amplitude, the processor detects a first received echo pulse whose amplitude exceeds the prescribed threshold value. The distance to an object is calculated on the basis of the detected first received echo pulse and the corresponding forward pulse. Immediately after the above-indicated first received echo pulse is detected, the forward-pulse transmission is suspended.
It is an object of this invention to provide a distance measurement apparatus having a good response characteristic.
A first aspect of this invention provides a distance measurement apparatus comprising electromagnetic wave generating means for generating and transmitting an electromagnetic wave; scanning means for periodically changing a direction in which the electromagnetic wave is transmitted from the electromagnetic wave generating means; receiving means for receiving an echo wave caused by reflection of the electromagnetic wave at an obstacle; first driving means for repetitively driving the electromagnetic wave generating means a plurality of times per one period of the change of the direction by the scanning means, and thereby for causing the electromagnetic wave generating means to repetitively generate and transmit a distance measurement electromagnetic wave; first calculating means for measuring a time interval between a moment of every generation and transmission of the distance measurement electromagnetic wave by the electromagnetic wave generating means in response to drive by the first driving means to a moment of reception of a corresponding echo wave by the receiving means, and for calculating a distance to an obstacle on the basis of the measured time interval; second driving means for, before the first driving means drives the electromagnetic wave generating means, driving the electromagnetic wave generating means and thereby causing the electromagnetic wave generating means to generate and transmit a judgment electromagnetic wave having an energy smaller than that of the distance measurement electromagnetic wave; and obstacle judging means for judging whether an obstacle is present or absent on the basis of conditions of reception of an echo wave corresponding to the judgement electromagnetic wave by the receiving means, for permitting the first driving means to drive the electromagnetic wave generating means next in cases where it is judged that an obstacle is absent, and for inhibiting the first driving means from driving the electromagnetic wave generating means next in cases where it is judged that an obstacle is present.
A second aspect of this invention provides a distance measurement apparatus comprising electromagnetic wave generating means for generating and transmitting an electromagnetic wave; scanning means for periodically changing a direction in which the electromagnetic wave is transmitted from the electromagnetic wave generating means; receiving means for receiving an echo wave caused by reflection of the electromagnetic wave at an obstacle; first driving means for repetitively driving the electromagnetic wave generating means a plurality of times per one period of the change of the direction by the scanning means, and thereby for causing the electromagnetic wave generating means to repetitively generate and transmit a distance measurement electromagnetic wave; first calculating means for measuring a time interval between a moment of every generation and transmission of the distance measurement electromagnetic wave by the electromagnetic wave generating means in response to drive by the first driving means to a moment of reception of a corresponding echo wave by the receiving means, and for calculating a distance to an obstacle on the basis of the measured time interval; second driving means for, before the driving means drives the electromagnetic wave generating means, driving the electromagnetic wave generating means and thereby causing the electromagnetic wave generating means to generate and transmit a judgment electromagnetic wave having an energy smaller than that of the distance measurement electromagnetic wave; and obstacle judging means for judging whether an obstacle is present or absent on the basis of conditions of reception of an echo wave corresponding to the judgement electromagnetic wave by the receiving means, for causing the first driving means to drive the electromagnetic wave generating means next to generate and transmit a first distance measurement electromagnetic wave in cases where it is judged that an obstacle is absent, and for causing the first driving means to drive the electromagnetic wave generating means next to generate and transmit a second distance measurement electromagnetic wave in cases where it is judged that an obstacle is present, wherein the second distance measurement electromagnetic wave is lower in energy than the first distance measurement electromagnetic wave.
A third aspect of this invention is based on the second aspect thereof, and provides a distance measurement apparatus wherein the second distance measurement electromagnetic wave is lower in amplitude than the first distance measurement electromagnetic wave.
A fourth aspect of this invention is based on the second aspect thereof, and provides a distance measurement apparatus wherein a duration of generation of the second distance measurement electromagnetic wave is shorter than that of the first distance measurement electromagnetic wave.
A fifth aspect of this invention is based on the second aspect thereof, and provides a distance measurement apparatus wherein each of the first distance measurement electromagnetic wave and the second distance measurement electromagnetic wave contains at least one pulse, and a pulse in the second distance measurement electromagnetic wave is smaller in width than that in the first distance measurement electromagnetic wave.
A sixth aspect of this invention is based on the second aspect thereof, and provides a distance measurement apparatus wherein each of the first distance measurement electromagnetic wave and the second distance measurement electromagnetic wave contains at least one pulse, and the second distance measurement electromagnetic wave is smaller in pulse number than the first distance measurement electromagnetic wave.
A seventh aspect of this invention is based on the second aspect thereof, and provides a distance measurement apparatus wherein the first distance measurement electromagnetic wave results from modulation in accordance with a pseudo noise code having a first bit length, and the second distance measurement electromagnetic wave results from modulation in accordance with a pseudo noise code having a second bit length smaller than the first bit length.
An eighth aspect of this invention is based on the second aspect thereof, and provides a distance measurement apparatus wherein the obstacle judging means comprises second calculating means for measuring a time interval between a moment of every generation and transmission of the judgement electromagnetic wave by the electromagnetic wave generating means in response to drive by the second driving means to a moment of reception of a corresponding echo wave by the receiving means, and for calculating a distance to an obstacle on the basis of the measured time interval, and judging means for judging whether an obstacle is present in or absent from a prescribed distance range on the basis of the distance calculated by the second calculating means.
A ninth aspect of this invention provides a distance measurement apparatus comprising first means for emitting a first laser beam in a first direction, the first laser beam having a first power; second means for receiving an echo corresponding to the first laser beam; third means for determining whether or not the second means receives an echo corresponding to the first laser beam; fourth means for emitting a second laser beam in the first direction in cases where the third means have determined that the second means receives an echo corresponding to the first laser beam, the second laser beam having a second power; fifth means for emitting a third laser beam in the first direction in cases where the third means have determined that the second means does not receive an echo corresponding to the first laser beam, the third laser beam having a third power, the third power being higher than the first power, the third power being higher than the second power; sixth means for emitting a fourth laser beam in a second direction after the fourth means emits the second laser beam or the fifth means emits the third laser beam, the second direction being different from the first direction, the fourth laser beam having the first power; seventh means for receiving an echo corresponding to the fourth laser beam; eighth means for determining whether or not the seventh means receives an echo corresponding to the fourth laser beam; ninth means for emitting a fifth laser beam in the second direction in cases where the eighth means have determined that the seventh means receives an echo corresponding to the fourth laser beam, the fifth laser beam having the second power; and tenth means for emitting a sixth laser beam in the second direction in cases where the eighth means have determined that the seventh means does not receive an echo corresponding to the fourth laser beam, the sixth laser beam having the third power.
A tenth aspect of this invention provides a distance measurement apparatus comprising first means for emitting a first laser beam in a first direction, the first laser beam having a first power; second means for receiving an echo corresponding to the first laser beam; third means for determining whether or not the second means receives an echo corresponding to the first laser beam; fourth means for emitting a second laser beam in the first direction in cases where the third means have determined that the second means does not receive an echo corresponding to the first laser beam, the second laser beam having a second power higher than the first power; fifth means for inhibiting the fourth means from emitting the second laser beam in cases where the third means have determined that the second means receives an echo corresponding to the first laser beam; sixth means for emitting a third laser beam in a second direction after the fourth means emits the second laser beam or the fifth means inhibits the fourth means from emitting the second laser beam, the second direction being different from the first direction, the third laser beam having the first power; seventh means for receiving an echo corresponding to the third laser beam; eighth means for determining whether or not the seventh means receives an echo corresponding to the third laser beam; ninth means for emitting a fourth laser beam in the second direction in cases where the eighth means have determined that the seventh means does not receive an echo corresponding to the third laser beam, the fourth laser beam having the second power; and tenth means for inhibiting the ninth means from emitting the fourth laser beam in cases where the eighth means have determined that the seventh means receives an echo corresponding to the third laser beam.